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(#) Missing @Keep for Animated Properties

!!! WARNING: Missing @Keep for Animated Properties
   This is a warning.

Id
:   `AnimatorKeep`
Summary
:   Missing @Keep for Animated Properties
Severity
:   Warning
Category
:   Performance
Platform
:   Android
Vendor
:   Android Open Source Project
Feedback
:   https://issuetracker.google.com/issues/new?component=192708
Since
:   2.3.0 (March 2017)
Affects
:   Kotlin and Java files and resource files
Editing
:   This check runs on the fly in the IDE editor
Implementation
:   [Source Code](https://cs.android.com/android-studio/platform/tools/base/+/mirror-goog-studio-main:lint/libs/lint-checks/src/main/java/com/android/tools/lint/checks/ObjectAnimatorDetector.kt)
Tests
:   [Source Code](https://cs.android.com/android-studio/platform/tools/base/+/mirror-goog-studio-main:lint/libs/lint-tests/src/test/java/com/android/tools/lint/checks/ObjectAnimatorDetectorTest.kt)

When you use property animators, properties can be accessed via
reflection. Those methods should be annotated with @Keep to ensure that
during release builds, the methods are not potentially treated as unused
and removed, or treated as internal only and get renamed to something
shorter.

This check will also flag other potential reflection problems it
encounters, such as a missing property, wrong argument types, etc.

!!! Tip
   This lint check has an associated quickfix available in the IDE.

(##) Example

Here is an example of lint warnings produced by this check:
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~text
src/main/java/AnimationExample.java:14:Warning: This method is accessed
from an ObjectAnimator so it should be annotated with @Keep to ensure
that it is not discarded or renamed in release builds [AnimatorKeep]
    public void setProp1(int x) {
                ---------------
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Here is the source file referenced above:

`src/main/java/AnimationExample.java`:
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~java linenumbers
import android.animation.ObjectAnimator;

public class AnimationExample {
    public void startAnimations() {
        Object myObject = new MyObject();
        ObjectAnimator animator1 = ObjectAnimator.ofInt(myObject, "prop1", 0, 1, 2, 5);
        animator1.start();

        ObjectAnimator animator2 = ObjectAnimator.ofInt(myObject, "prop2", 0, 1, 2, 5);
        animator2.start();
    }

    private static class MyObject {
        public void setProp1(int x) {
            // Implementation here
        }

        private void setProp2(float x) {
            // Implementation here
        }
    }
}
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

You can also visit the
[source code](https://cs.android.com/android-studio/platform/tools/base/+/mirror-goog-studio-main:lint/libs/lint-tests/src/test/java/com/android/tools/lint/checks/ObjectAnimatorDetectorTest.kt)
for the unit tests for this check to see additional scenarios.

(##) Suppressing

You can suppress false positives using one of the following mechanisms:

* Using a suppression annotation like this on the enclosing
  element:

  ```kt
  // Kotlin
  @Suppress("AnimatorKeep")
  fun method() {
     ofInt(...)
  }
  ```

  or

  ```java
  // Java
  @SuppressWarnings("AnimatorKeep")
  void method() {
     ofInt(...);
  }
  ```

* Using a suppression comment like this on the line above:

  ```kt
  //noinspection AnimatorKeep
  problematicStatement()
  ```

* Adding the suppression attribute `tools:ignore="AnimatorKeep"` on
  the problematic XML element (or one of its enclosing elements). You
  may also need to add the following namespace declaration on the root
  element in the XML file if it's not already there:
  `xmlns:tools="http://schemas.android.com/tools"`.

  ```xml
  &lt;?xml version="1.0" encoding="UTF-8"?&gt;
  &lt;android.support.constraint.motion.MotionLayout xmlns:tools="http://schemas.android.com/tools"
      tools:ignore="AnimatorKeep" ...&gt;
    ...
  &lt;/android.support.constraint.motion.MotionLayout&gt;
  ```

* Using a special `lint.xml` file in the source tree which turns off
  the check in that folder and any sub folder. A simple file might look
  like this:
  ```xml
  &lt;?xml version="1.0" encoding="UTF-8"?&gt;
  &lt;lint&gt;
      &lt;issue id="AnimatorKeep" severity="ignore" /&gt;
  &lt;/lint&gt;
  ```
  Instead of `ignore` you can also change the severity here, for
  example from `error` to `warning`. You can find additional
  documentation on how to filter issues by path, regular expression and
  so on
  [here](https://googlesamples.github.io/android-custom-lint-rules/usage/lintxml.md.html).

* In Gradle projects, using the DSL syntax to configure lint. For
  example, you can use something like
  ```gradle
  lintOptions {
      disable 'AnimatorKeep'
  }
  ```
  In Android projects this should be nested inside an `android { }`
  block.

* For manual invocations of `lint`, using the `--ignore` flag:
  ```
  $ lint --ignore AnimatorKeep ...`
  ```

* Last, but not least, using baselines, as discussed
  [here](https://googlesamples.github.io/android-custom-lint-rules/usage/baselines.md.html).

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